The Reaction of Triplet Flavin with Indole. A Study of the Cascade of Reactive Intermediates Using Density Functional Theory and Time Resolved Infrared Spectroscopy

Abstract

As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical[bond]radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the pi radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N(5). A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF* radical can then undergo radical[bond]radical coupling reactions, with the most thermodynamically stable adduct being formed at C(4'). Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B2)

“…Among all the isomeric flavin semiquinone neutral radicals formed upon protonation of the radical anion, the energy of N(5)-protonation structure is the lowest. [14] Electron spin resonance (ESR) spectroscopy also gives similar results on comparison of the hyperfine coupling constants. [37] Due to the special structure of DHLA, the mercaptothiyl radical LipSSH .…”

“…[6,8,11] Charge density analysis indicates that the flavin radical anion exhibits a significant increase in negative charge density at N(5) and C(4') compared to the ground state of flavin. [14] Spin density analysis demonstrates that there is a localization of spin density at N(5) and C(4'). This observation indicates that the single unpaired electron in the flavin radical anion is substantially delocalized between C(4')-N(5) (Scheme 1).…”

“…By calculating the spin density of triplet flavin with DFT methods, Platz and co-workers recently found that this species can be best described as a biradical with unpaired electrons centered at the N(1) and N(5) atoms. [14] The steadystate UV/visible absorption spectra of LipSS and DHLA at pH 7.5 in aqueous solution are shown in Figure 1. Due to the strained five-membered-ring disulfide structure, LipSS absorbs at around 330 nm.…”

Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

“…The JASCO TRIR-1000 dispersive spectrometer used to record ns TRIR difference spectra has been described previously [25]. Briefly, samples were held in a home-built flow cell composed of two 1 cm thick CaF 2 plates with a 0.5 mm Teflon spacer and excited using 266 nm pulses (~0.7 mJ / pulse, repetition rate of 97 Hz) from the fourth harmonic of an Infinity Nd:YAG laser (Coherent, Inc).…”

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

“…The experimental studies have been assisted by various ab initio calculations. [34][35][36][37][38][39][40] In many cases, the first step of the photo-initiated dynamics consists of an ultrafast electron transfer from a donor to the excited flavin.…”